fix #123531: separate Flemish from Dutch Qt translation

[Jojo-Schmitz]

for the difference in translation of the "Discard" button, 'Verwerpen' vs. 'Niet opslaan'. Also add shortcuts to the Discard and Save buttons.
Also make sure the qt_*.qm files get build by 'make lrelease'

[jeetee]

+0 from a Flemish voice; our languages share nearly all terms with regards to software linguistics.

Further looking into this specific one, it seems Dutch people mostly see 'Verwerpen' as a dated translation, although it is technically still the correct translation of "Discard" vs "Don't Save" (reference to a similar question (in Dutch) for the OpenOffice project: https://forum.openoffice.org/nl/forum/viewtopic.php?f=16&t=399)
I personally don't find the minor differences between our languages big enough to warrant separate translations. I'm also not opposed to changing the translation to match the modern trend, but looking at that evolution it seems more programs are tending towards a simplified Yes/No/Cancel, rather than Save/Discard/Cancel